1. Field of the Invention
The present invention relates to a pyroelectric device for detecting infrared radiation which is for use in a flame sensor for fire detection or in an intrusion detector used for prevention of crimes.
2. Description of the Related Art
FIG. 6 shows the structure of a conventional pyroelectric device. A casing includes a metal can 10 and a stem 11 welded to the can 10. The metal can 10 is hermetically sealed by a filter 9 made of silicon. Three pins 4, 5 and 6 respectively constituting a drain terminal, a source terminal and a grounding terminal pass through the stem 11. A printed-circuit board 12 is placed on and fixed to the upper end portions of the pins 4, 5 and 6 by soldering or the like to construct an electric circuit portion.
In the electric circuit of the pyroelectric device, the source electrode of a field-effect transistor (hereinafter referred to as an FET), serving as a preamplifier, is connected to one of electrodes of the pyroelectric member. In order to control the frequency characteristics of the detection sensitivity, a gate resistor is parallel-connected to the pyroelectric member. The electric time constant of the gate resistor determines the cut-off frequency in the low frequency band. A gate resistor having a high resistance of 100 to 200 gigaohms is used, and especially for detection of a human body the pyroelectric device itself has a wider frequency characteristic.
If a gate resistor having a resistance of, for example, 100 gigaohms is used, the sensitivity of the pyroelectric device with a response frequency characteristics curve of arch-form having a peak at about 0.5 Hz is obtained. The frequency characteristics of the infrared radiation from a human body which are required for an intrusion detector are peaked at about 0.1 Hz. The frequency characteristics of a flame required for a fire alarm sensor are peaked at about 8 Hz. Thus, if the above-described response frequency characteristics peaked at about 0.5 Hz are employed for detection of a flame, the peak of the output sensitivity cannot be obtained in the desired frequency band.
Furthermore, where the ambient temperature of the pyroelectric device rapidly changes, the pyroelectric device may not be able to produce an output. That is, when the resistance of the gate resistor is high, the voltage applied to the pyroelectric member is low. Therefore, if the ambient temperature drops at a rate of, for example, 1.4.degree. C./min, a nonsignal output state (a pinch-off state) results. Changes in the ambient temperature may depend on the installation site of the sensor or detector. A detector which is in a nonsignal state causes a serious problem.